For the presentation of different transmission ratios, transmissions of vehicle drive trains known from practice are typically formed with positive-locking and/or frictional-locking shifting elements for the connecting and disconnecting of transmission ratios. Positive-locking shifting elements are typically designed as claw shifting elements, which are subjected to actuating pressure through claw valves by hydraulic actuating devices of the transmission. Viewed physically, such claw valves represent so-called “hydraulic switches,” through which the actuating pressure can be driven in the direction of a shifting element within short periods of operation, or the actuation of a shifting element can be switched off to the desired extent with short control and regulating periods. Upon the actuation of such claw valves, very high hydraulic performance can be withdrawn from a hydraulic pump provided with a hydraulic fluid for a determinable period of time, for example for 10 to 20 milliseconds. Through the high degree of withdrawal, under normal circumstances, as described in more detail below, there is an occurrence of unwanted excitations in the curve of the system pressure, typically in the form of pressure spikes, which may result in damages or a failure of sealing devices, in addition to a failure of the hydraulic pump. In addition, through the triggered changes to clutch pressures, converter pressures and the like, damages and failure symptoms in the area of connecting lines and the claw shifting elements themselves, along with undesired influences of other hydraulically supplied system components, are possible.
If there is a request for actuating a preferably positive-locking shifting element, with the approach known from practice, an actuating pressure or system pressure level is initially raised to the pressure level required for the displacement of the shifting element. Typically, the shifting element is subjected to the maximum system pressure, in order to, as requested, engage the shifting element or the claw in the shortest possible time. This is necessary, since positive-locking shifting elements typically feature the operating state necessary for the switching on of a claw shifting element, i.e. an essentially load-free and nearly synchronous operating state, only within a limited time window. If the actuating pressure level necessary for the switching on of the positive-locking shifting element or the claw shifting element, as the case may be, is reached, the control command is carried out for the claw valve, preferably designed as a solenoid valve, which switches over after the expiration of a delay period for approximately 10 milliseconds. After the switching over of the claw valve, the actuating pressure is applied in the area of a pressure chamber of the claw shifting element. This in turn leads to the fact that the claw shifting element or an actuating piston of the claw shifting element is displaced starting from its first end position in the direction of its second end position. Thereby, the volume of the pressure chamber subjected to actuating pressure and limited by the actuating piston of the claw shifting element increases, and the hydraulic fluid volume flowing into the pressure chamber creates a high flow disturbance in the area of the hydraulic actuating device of a transmission that drives system pressure, and causes a short-term decline in the curve of the system pressure.
If the actuating piston reaches its second end position and abuts against the end stop, a pressure peak occurs in the system pressure curve, which in turn leads to a high excitation of the hydraulic system or the hydraulic actuating device, as the case may be. Due to the high degree of excitation, a transient oscillation in the direction of the actuating pressure level requested at the beginning of the displacement operation of the claw shifting element is adjusted with additional pressure peaks, the amplitudes of which are smaller than the amplitude of the pressure peak that occurs upon reaching the second end position of the actuating piston.
It is thereby problematic that a system pressure release valve, in the area of which the system pressure is adjusted, reacts to the decline in system pressure that arises during the displacement of the actuating pressure and reduces the associated throttle cross-section in order to increase the system pressure. For this reason, upon reaching the second end position of the actuating piston of the claw shifting element, the system pressure valve is throttled too strongly, in order to compensate for the occurrence of undesirably high pressure peaks upon reaching the second end position to a sufficient extent.
In order to avoid such excitations in the system pressure curve, which impair the functioning of a transmission on a long-term basis, there is, for example, the possibility of providing a highly dynamic, fast-switching safety valve in the area driving the system pressure of a hydraulic system of a transmission device.
However, it is thereby disadvantageous that such safety valves present additional component cost and must be designed in an accordingly large size in order to implement the desired high dynamics.
Additional capacities in the form of accumulators in the area of the transmission driving the system pressure represent an additional possibility for avoiding undesired pressure peaks or excitations in the system pressure. Such pulsation dampers are sometimes made of plastic, and are installed directly in the area of the hydraulic pump. Notwithstanding this, there are also traditional spring-piston accumulators, by means of which there is compensation for pressure peaks.
However, it is disadvantageous that such solutions are once again characterized by increased component cost. Furthermore, the spring-piston accumulator provided for reducing pressure peaks in the area driving the system pressure impairs system pressure dynamics, since the function of the spring-piston accumulator lowers spontaneity in control, due to a slower build-up of system pressure.
With an additional approach known from practice, for actuating a positive-locking shifting element of a transmission, upon the presence of a request for actuating the positive-locking shifting element in the area of a piston of the shifting element, which is designed to be displaceable between a first end position and a second end position, an actuating pressure of the positive-locking shifting element is applied, and the actuating piston is, depending on the current request, thereby guided in the direction of its first or its second end position, which corresponds to a closed operating state of the shifting element or to an open operating state of the shifting element, as the case may be.
The actuating pressure applied at the actuating piston is reduced prior to reaching the first end position or prior to reaching the second end position of the actuating piston, and, upon reaching the first end position or upon reaching the second end position of the actuating piston, in comparison with the approaches described above, an excitation in the curve of the actuating pressure is reduced. Thus, damages or a failure of a hydraulic pump, sealing devices, the connecting lines and/or a positive-locking shifting element or a claw shifting element itself, along with undesired influences in other hydraulically supplied system components of a transmission, are avoidable.
However, within the framework of the displacement speeds of the actuating piston that arise during the actuation of a claw shifting element and the usual sampling times in the transmission, this approach is often difficult to carry out, particularly since an actuating pressure applying at the actuating piston is optimally reduced in a corresponding measure shortly prior to the stopping of the piston at the mechanical stop, in order to not prematurely dampen the actuation of the claw shifting element.